1. Field of the Invention
The present invention relates to injection locked magnetrons and, more particularly, to a high impedance anode structure utilizing a novel vane configuration.
2. Description of Related Art
Magnetrons have been used for several years in electronic systems that require high RF power, such as radar systems. A magnetron typically includes a central cylindrical shaped cathode coaxially disposed within an annular anode structure with an interaction region provided between the cathode surface and the anode. The anode structure may include a network of vanes which provides a resonant cavity tuned to provide a mode of oscillation for the magnetron.
Upon application of an electric field between the cathode and the anode, the cathode surface emits a space-charge cloud of electrons. A magnetic field is provided along the cathode axis, perpendicular to the electric fields, which causes the emitted electrons to spiral into cycloidal paths in orbit around the cathode. When RF fields are present on the vane structure, the rotating space-charge cloud is concentrated into a spoke-like pattern. This is due to the acceleration and retardation of electrons in regions away from the spokes. The electron bunching induces high RF voltages on the anode circuit, and the RF levels on the anode build up until the magnetron is drawing full peak current for any given operating voltage. Electron current flows through the spokes from the cathode to the anode, producing a high power RF output signal at the desired mode of oscillation.
One particular type of magnetron, known as an injection locked magnetron, utilizes an external oscillator to inject a sinusoidal signal into the anode structure of the magnetron at a frequency close to its natural resonant frequency. These injection locked magnetrons can then be caused to operate in the .pi. mode of oscillation at a precise frequency determined by the external oscillator. The advent of higher power solid state oscillators has increased the feasibility of injection locked magnetrons. Injection locked magnetrons are further described in U.S. Pat. No. 5,045,814, by English et al., which is assigned to the common assignee, and which is incorporated herein by reference.
It has long been desirable in magnetrons to increase the size of the cathode so as to increase the output power of the magnetron. Enlarging the cathode would enable the magnetron to produce the same amount of power while decreasing the current density on the cathode surface, known as cathode loading. The lower the cathode loading, the longer the lifetime of the cathode. Since cathode degradation is a significant cause of magnetron failure, it is highly desirable to increase the life of the cathode. In addition, reducing the cathode loading would reduce the thermal loading on the anode structure, further improving the reliability and life of the magnetron.
A significant problem with this approach is that it has not been possible to build a large diameter cathode magnetron in actual practice. Traditionally, magnetrons have a limited number of anode vanes, such as twelve or eighteen, which form the resonant cavity and determine the modes of oscillation. As the cathode diameter increases, the anode diameter also needs to increase. This causes the distance between adjacent vane tips proximate to the cathode surface to become too large, and the orbiting electrons would not be synchronized to the RF field. As a result, the magnetron will no longer oscillate at the desired peak power level.
To keep the adjacent vane tips at acceptable distances for proper oscillation to occur, a large diameter cathode magnetron would require a higher number of anode vanes. However, as the number of vanes is increased, the overall impedance of the anode structure decreases and the magnetron becomes unstable. The mode separation becomes so small that oscillation cannot be maintained at a desired mode. For these reasons, magnetrons having greater than 30 anode vanes are generally considered impractical. If the impedance of the anode structure could be maintained at a high level, the number of anode vanes could be increased and the cathode diameter could be enlarged.
Accordingly, a need exists to provide an anode structure for a magnetron having a relatively high impedance to permit an increased number of anode vanes. Ideally, the anode structure would provide increased mode separation over conventional magnetrons.